Hyperthyroid Disease in Cats

By: Dr. Amy Plankenhorn

Last spring, Susan Key noticed that her 11-year-old cat Griffin was becoming a lot more talkative. He was howling every morning and begging for food.  She had struggled with keeping his excess weight controlled for a long time, but he started unexpectedly losing weight despite a big appetite. His annual comprehensive physical exam came due, and sure enough, he had lost a pound since his previous visit. His heart rate was elevated. Blood testing confirmed our suspicions – he had hyperthyroidism.

Hyperthyroid disease is a common disorder of cats. Typically it is seen in cats over 10 years of age, although occasionally younger cats can develop the disease. Affected cats develop a benign nodule on the thyroid gland, which causes overproduction of thyroid hormone. Thyroid hormone is responsible for maintaining a normal rate of metabolism in the body, so overproduction typically leads to weight loss, ravenous appetite, increased thirst, increased urination, vomiting, increased activity, or increased vocalization. Their heart rates are often very rapid, and they sometimes have heart murmurs detected during physical examination. We see cats who do not fit all of the typical signs and are diagnosed early as a result of annual blood testing.

We recommend annual wellness screening blood tests starting from kittenhood, and twice a year for cats over 10 years of age. The panel includes a screening thyroid test. If a cat’s symptoms are consistent with hyperthyroid disease, but the screening test is on the borderline, we may repeat the test in a few months. But the diagnosis is clear in most cats.

Untreated, thyroid disease can cause a variety of problems. Weight loss is the most visible problem that hyperthyroid cats exhibit. Constant hunger, thirst, vomiting, and restlessness are all uncomfortable for cats. Hyperthyroid cats can develop high blood pressure, which can damage the kidneys and even cause retinal detachment and blindness. The rapid heart rate caused by the high metabolism can lead to eventual congestive heart failure. A cat who is constantly begging for food, or who is more restless or vocal can be hard for owners to live with, too.

This is a different cat. He has untreated hyperthyroid disease. He is very thin, anxious, and his fur is matted.

Early diagnosis and treatment of thyroid disease is the best way to prevent the visible and less visible complications of the disease. Fortunately, there are several treatment options available for hyperthyroid cats.  We usually start hyperthyroid cats on an oral medication called Tapazole, which blocks the production of the thyroid hormone. We repeat blood tests 2-4 weeks after starting the medication to make sure we are controlling the thyroid levels and that there are no other underlying health problems. Some cats are able to stay on Tapazole long-term, but this is a lifelong treatment. More definitive treatment is accomplished with radioactive iodine therapy. The thyroid gland requires iodine to produce its hormone, so by tagging iodine with a radioactive substance, the thyroid can be treated with gland-specific radiation therapy with no danger to the cat. The treated cat has to stay in the hospital for 3-5 days because of the radioactive material in the urine, but the treatment is safe and painless.This treatment is performed in several referral centers in the North and South Carolina.

Griffin started his treatment with Tapazole, but it upset his stomach. He received radioactive iodine therapy last spring, and is back to his normal self again. Fortunately, follow up blood testing has shown good control of his thyroid levels a year later. His sister Sabine, who had normal blood tests last year, was diagnosed with early stage hyperthyroid disease this spring during her annual wellness testing, and has recently had radioactive iodine therapy.  Thanks to Susan's commitment to regular monitoring and prompt treatment, we are optimistic that Griffin and Sabine can continue to live long, healthy lives.